1. Field of the Invention
This invention relates to a bicycle multiple sprocket assembly having at least two diametrically different sprockets. More specifically, the invention relates to a multiple sprocket assembly which is designed to provide smooth chain shifting from a larger sprocket to a smaller sprocket.
The term "multiple sprocket assembly" as used herein includes a multiple chainwheel (front gear) which is mounted on a pedal crank assembly, and a multiple freewheel (rear gear) mounted on a rear wheel hub.
2. Description of the Prior Art
Many of currently available bicycles not only have a multiple freewheel to constitute a rear gear mounted on a rear wheel hub, but also incorporate a multiple chainwheel to constitute a front gear mounted on a pedal crank, thereby increasing the number of selectable speeds. A selected sprocket of the front gear is connected to a selected sprocket of the rear gear by an endless chain, and the pedalling force applied to the front gear is transmitted through the chain to the rear gear for driving the bicycle forward.
With a multiple sprocket assembly, a speed change is performed by causing a derailleur (front derailleur or rear derailleur) to laterally press a portion of the chain entering to the sprocket assembly in rotation, which results in that the chain is laterally inclined for shifting from a smaller sprocket to a larger sprocket or vice versa. Obviously, the speed change performance of the sprocket assembly is determined by the smoothness and promptness in disengaging the chain from a presently engaging sprocket for engagement with a target sprocket.
In shifting the chain from a larger sprocket to a smaller sprocket, the chain need only be disengaged from the larger sprocket because a tension applied to the chain can be utilized for causing the chain to automatically fall into engagement with the smaller sprocket. For this reason, it has been hitherto considered easier to shift the chain from the larger sprocket to the smaller sprocket than to shift it from the smaller sprocket to the larger sprocket. Therefore, not much effort has been made to improve chain shiftability from the larger sprocket to the smaller sprocket.
In reality, however, each sprocket of a multiple sprocket assembly has a different number of teeth. Thus, the chain disengaging from the larger sprocket is not necessarily positioned suitably relative to the teeth of the smaller sprocket. In some cases, the chain (chain rollers) may ride on the teeth apexes of the smaller sprocket and move idly through a small angle before completely coming into driving engagement with it. As a result, a large drive force is abruptly applied to the smaller sprocket and the chain, thereby damaging these components. Further, the rider feels a considerable shock at the time of this phenomenon, which sometimes causes danger to the rider.
In view of the above problem, Japanese Utility Model Application Laid-open No. 64-40791 discloses an improved multiple sprocket assembly which is designed so that the chain is made to start shifting from a larger sprocket to a smaller sprocket at a selected angular position of the sprocket assembly. Further, the relative angular position of the smaller and larger sprockets is suitably adjusted so that the chain engages the smaller sprocket without idle movement when the chain starts disengaging at the selected position.
Specifically, the larger sprocket of the above Japanese application comprises an easily disengageable tooth (or a plurality of such teeth) to insure that the chain starts disengaging at this tooth (corresponding to the selected angular position). Further, the smaller and larger sprockets are angularly adjusted so that a tangential line drawn from the furrow center immediately following the easily disengageable tooth to a furrow center of the smaller sprocket has a length which is slightly smaller than np (n: integer number; p: chain pitch). Thus, the chain disengaging from the larger sprocket at the easily disengageable tooth can come immediately into driving engagement with the smaller sprocket teeth without idle movement.
However, the multiple sprocket assembly of the Japanese application is still disadvantageous in the following points.
First, when the chain disengages from the larger sprocket at the easily disengageable tooth, the disengaging chain interferes laterally with the easily disengageable tooth and another tooth immediately following the easily disengageable tooth. Such lateral interference causes the disengaging chain t be laterally inclined or bent to an excessive degree under the tension applied to the chain. Thus, the chain and the larger sprocket are mechanically damaged by repetitive shifting of the chain from the larger sprocket to the smaller sprocket, consequently shortening the life of these components.
Secondly, excessive lateral inclination of the disengaging chain causes it to skip the smaller sprocket, thereby failing to perform intended chain shifting. This problem becomes particularly pronounced when the axial spacing between the smaller and larger sprockets is relatively small.